1. Technical Field
The present invention generally relates to a compact DC (direct-current) motor suited for use in audio-visual appliances such, for example, as optical pickups for MDs or CD-ROMs and, in particular but not exclusively, to a permanent magnet field-type compact DC motor utilizing arcuate permanent magnets prepared from rare earth metal, having a low cogging torque, capable of providing a high output and capable of precisely rotating with minimized starting voltage and current. The present invention also relates to a method of making the permanent magnet field-type compact DC motor.
2. Description of Related Art
FIGS. 1A and 1B depict a permanent magnet field-type compact DC motor and an arcuate permanent magnet used therein, respectively, to which the present invention is applicable. In these figures, reference numeral 1 represents a pair of arcuate permanent magnets; reference numeral 2 represents a soft magnetic frame; reference numeral 3 represents an armature including a brush-commutator, an armature shaft and a bearing; and reference numeral 4 represents generally U-shaped springs used to urge and fixedly retain the respective arcuate permanent magnets 1 against the soft magnetic frame 2. Even this permanent magnet field-type compact DC motor is, as is the case with any other permanent magnet motor, required to have a compact size, a high output and a highly precise rotational performance.
However, it has been recognized that the permanent magnet field-type compact DC motor has a problem in that the use of a reduced diameter of the armature 3 renders it difficult to reduce the size of the motor without the output performance being sacrificed. Specifically, it is well known that ferrite magnets generally have a relatively low maximum energy product [BH]max regardless of the compression, injection or extrusion molding process using a sintering technique or a resinous material as a binder. Therefore, reduction in size of the ferrite magnets used in the permanent magnet field-type compact DC motor would result in that no sufficient static magnetic field is developed in a space between the permanent magnets 1 and the armature 3, thus considerably reducing the motor output. In view of this, arcuate rare earth magnets having a so-called high maximum energy product [BH]max have long been desired for, because they can provide a strong static magnetic field in the space between the permanent magnets 1 and the armature 3 even though the motor is reduced in size.
When it comes to the arcuate rare earth magnets of a maximum usable thickness smaller than 1 mm, which are effective to reduce the size of the permanent magnet field-type compact DC motor, the method of making such arcuate rare earth magnets is currently limited. By way of example, Japanese Laid-open Patent Publication (unexamined) No. 6-236807 discloses a method of making an arcuate rare earth magnet by the use of an extrusion molding technique. According to this known method, a molten fluidized material including various rare earth magnetic powder having a characteristic ranging from a magnetically anisotropy to a magnetically isotropy in admixture with a thermoplastic resin is poured into a mold assembly and is, after having been cooled within the mold assembly down to a temperature lower than the melting point of the thermoplastic resin, extruded to form an arcuate rare earth magnet. The resultant arcuate rare earth magnet is described as having a maximum thickness of 0.9 mmxc2x130 xcexcm.
On the other hand, the permanent magnet field-type compact DC motor utilizing the arcuate rare earth magnets of the kind described above and capable of providing a strong static magnetic field in the space between the rare earth magnets and the armature 3 as compared with the ferrite magnets has an additional problem associated with increase of the cogging torque. Because of the presence of armature iron core teeth 31 and slots 32 on an outer peripheral surface of the armature 3 facing the permanent magnets 1, the cogging torque results from generation of torque pulsation brought about by change in permeance incident to rotation of the armature 3. The cogging torque is detrimental to the permanent magnet field-type compact DC motor that is required to have a reduced size, a high output and a highly accurate rotational performance for which the present invention is intended.
Regardless of whether or not the permanent magnets used are in the form of rare earth magnets, means for reducing the cogging torque defined by the shape of the arcuate magnets employed in the permanent magnet field-type compact DC motor is implemented by causing inner and outer radii of curvature of the arcuate magnets to be eccentric or by chamfering edges of circumferentially spaced, opposite end faces of the arcuate magnets to render them to have an inequality wall thickness so that the permeance of the magnets, in which a uniform material is uniformly magnetized from the center of a magnetic pole to any of opposite ends of each magnet, can change so as to render the distribution of void magnetic flux densities to represent a generally sinusoidal shape. (See, for example, Shogo Tanaka, xe2x80x9cKogata Mohta-niokeru Eikyuujishaku no Ouyou (Application of Permanent Magnets in Compact Motors)xe2x80x9d, transaction of Kogata Mohta Gijutsu Symposium, pp. 7, 1983.)
On the other hand, U.S. Pat. No. 4,710,239 discloses a method of making an arcuate magnet. According to this disclosure, as the starting material, there is used rare earth-iron-based rapidly quenched and solidified amorphous flakes of a composition comprising, on an atomic basis, 10 to 50% of rare earth element Re (Nd and/or Pr), 1 to 10% of B, the balance being transition metal element TM, at least 60% of the transition metal element being Fe. This starting material is compacted at a temperature higher than the crystallizing temperature (about 600xc2x0 C.) of a magnet phase RE2TM14B, but lower than 750xc2x0 C. to form an isotropic fully dense magnet having a different thickness, followed by hot plasticizing process at similar temperature to provide the arcuate magnet. Since depending on the extent to which the hot plasticization is effected, easy axes of magnetization are oriented in a direction perpendicular to the direction of plastic flow, RE2TM14B can be transformed into an anisotropic rare earth magnet having a strong magnetism in a relatively thick portion thereof. Thus, the above mentioned USP discloses the arcuate magnet having portions exhibiting a strong anisotropy and an isotropy, respectively.
If the arcuate rare earth magnets each having the portions of different magnetic performances are used in the permanent magnet field-type DC motor, it is possible to vary a demagnetization curve in a circumferential direction from the center of the magnetic pole and, therefore, it is also possible to render the distribution of the void magnetic flux densities between the field magnets and the armature iron core to vary smoothly to reduce the cogging torque.
As discussed above, the use in the permanent magnet field-type DC motor may be made of the arcuate rare earth magnets each having the inner and outer radii of curvature that offset relative to each other so as to render them to have a varying wall thickness, or of the arcuate magnets with their circumferential edges chamfered to have a varying wall thickness, so that the permeance of the magnets can change from the center of the magnetic pole towards the circumferentially opposite ends and, also, the magnetism of the magnets can be varied in a circumferential direction from the center of the magnetic pole, to control the pattern of distribution of the void magnetic flux densities between the field magnets and the armature iron core so as to allow the cogging torque to reduce due to the different demagnetization curves.
However, the arcuate rare earth magnets of a thickness-smaller than 1 mm suited for use in the permanent magnet field-type compact DC motor to which the present invention is applicable, each of which magnets is prepared by hot working the fully dense magnet having a different thickness with the use of the starting material in the form of the rare earth-iron-based rapidly quenched and solidified amorphous flakes and has portions of the different magnetic performances, are extremely difficult to manufacture on an industrial scale. Also, if the anisotropy is strengthened, the temperature dependent coefficient of the intrinsic coercive force tends to be reduced. For these reasons, a problem tends to arise that thermal demagnetization at the center of the magnetic pole having the strengthened anisotropy is substantially considerable, accompanied by considerable reduction in torque as a result of the thermal demagnetization of the permanent magnet field-type DC motor.
According to the method disclosed in the previously discussed Japanese publication, a material containing, for example, 95 wt % of magnetically isotropic, rare earth-iron-based rapidly quenched and solidified flakes and a thermoplastic resin including Nylon-12 as a principal component is used to produce by the use of an extrusion molding technique the thin-walled arcuate rare earth magnet having a maximum thickness of 0.9 mmxc2x130 xcexcm. However, the extrusion molding requires the thermoplastic resin when in a molten state to serve as a carrier for the rare earth-iron-based rapidly quenched and solidified flakes.
Accordingly, as compared with the rare earth magnet formed by compacting the rare earth-iron-based rapidly quenched and solidified flakes mixed with generally 3 or smaller wt % of a thermosetting resin, the amount of such flakes to be filled must be reduced, accompanied by corresponding reduction of [BH]max, thus reducing the static magnetic field in the gaps between the field magnets 1 and the armature 3. Although it may be contemplated to make the arcuate magnet having a high [BH]max by the use of a magnetically anisotropic magnetic powder, the prime problem which the present invention is intended to solve lies in the difficulty involved in the compacting technique in making the thin-walled arcuate magnet of a maximum thickness smaller than 1 mm within a tolerance of xc2x130 xcexcm due to variation in scale during the molding (see Japanese Laid-open Patent Publication No. 6-236807 referred to above). It is, however, pointed out that even though this problem is successfully solved, the resultant arcuate magnet formed by the compacting technique is mechanically fragile to an extent the amount of the resin is reduced.
Accordingly, the arcuate rare earth magnet formed by the use of the compacting technique cannot be deflected to snugly fit to a corresponding portion of the soft magnetic frame when it is to be fixedly positioned such as described in Japanese Laid-open Patent Publications (unexamined) No. 10-201206 and No. 11-18390. In other words, the secondary problem to be solved by the present invention lies in means for mounting the arcuate rare earth magnets, formed by the use of the compacting technique, to the soft magnetic frame in a manner consistent with the mechanical nature of those magnets. Also, since as compared with the arcuate rare earth magnets formed by the use of the extrusion molding technique, the arcuate rare earth magnets tend to have a high [BH]max, the third problem to be solved by the present invention is to provide a method of making the permanent magnet field-type compact DC motor of a kind wherein, along with the cogging torque reducing means brought about by the well known shape of the magnets, the arcuate rare earth magnets molded from the rare earth-iron-based rapidly quenched and solidified flakes of a magnetically isotropic nature into an arcuate shape are unsaturation-magnetized to exhibit different demagnetization curves at the center of the magnetic pole and at the circumferentially opposite ends thereof to thereby reduce the cogging torque.
The present invention is intended to provide a permanent magnet field-type compact DC motor capable of providing a stronger static magnetic field in spaces between an armature and field magnets than the ferrite magnets, and having a low cogging torque, a small size and a high output. The present invention is also intended to provide a method of making such a permanent magnet field-type compact DC motor. The DC motor according to the present invention is made by molding rare earth-iron-based rapidly quenched and solidified flakes of a magnetic isotropy into an arc.
In accomplishing the above and other objectives, the method of making a permanent magnet field-type compact DC motor according to the present invention comprises the steps of fixing a pair of rare earth magnets mainly composed of rare earth-iron-based rapidly quenched and solidified flakes to a soft magnetic frame so as to extend along an inner peripheral surface thereof, and unsaturation-magnetizing the rare earth magnets so that demagnetization curves at circumferentially opposite end portions of the rare earth magnets are made smaller than a demagnetization curve at a central portion of a magnetic pole of each of the rare earth magnets.
Advantageously, the soft magnetic frame has portions that do not serve as a back yoke at outer peripheral surfaces of the circumferentially opposite end portions of the rare earth magnets, and the rare earth magnets are unsaturation-magnetized with a soft magnetic material having a round or oval section interposed therebetween.
Alternatively, spaces are formed on respective sides of a magnetizing yoke and between the magnetizing yoke and an outer peripheral surface of the soft magnetic frame at locations of the circumferentially opposite end portions of the rare earth magnets, and the rare earth magnets are unsaturation-magnetized with a soft magnetic material having a round or oval section interposed therebetween. The rare earth magnets are unsaturation-magnetized by creating a greater demagnetizing field at the circumferentially opposite end portions of the rare earth magnets than at the central portion of the magnetic pole.
Again advantageously, the circumferentially opposite end portions of each of the arcuate rare earth magnets have respective generally flat surfaces forming an angle of 50xc2x0 to 82xc2x0 on an outer peripheral surface thereof so that the soft magnetic frame does not serve as a back yoke at the circumferentially opposite end portions of the arcuate rare earth magnets.
It is preferred that the strength of an unsaturation-magnetizing field ranges from 15 to 30 kOe.
After the pair of rare earth magnets have been unsaturation-magnetized, they are heated so that a demagnetizing factor at the circumferentially opposite end portions is made greater than that at the central portion of the magnetic pole.
The rare earth-iron-based rapidly quenched and solidified flakes are made of a magnetically isotropic RE2TM14B (RE: Nd or Pr; TM: Fe or Co) phase smaller than 300 nm and have an intrinsic coercive force of 8 to 10 kOe and a remanent magnetization of 7.4 to 8.6 kG.
The rare earth-iron-based rapidly quenched and solidified flakes contain magnetically isotropic ones of a nano-composite structure having a soft magnetic phase such as xcex1Fe, Fe3B, Fe2B or the like and a hard magnetic phase such as RE2TM14B.
The rare earth magnets are obtained by compression-molding the rare earth-iron-based rapidly quenched and solidified flakes with a bonding agent, and the circumferentially opposite end portions of each of the rare earth magnets are urged and fixedly retained against the soft magnetic frame by respective springs. Each of the springs is made of a soft magnetic material having a round or oval section.
Alternatively, the rare earth magnets are obtained by extrusion-molding the rare earth-iron-based rapidly quenched and solidified flakes with a bonding agent, and the circumferentially opposite end portions of each of the rare earth magnets are caused to engage with respective stoppers formed on the soft magnetic frame.
The rare earth magnets may be fully dense magnets obtained by hot compression-molding the rare earth-iron-based rapidly quenched and solidified flakes at a temperature higher than a crystallizing temperature and lower than 750xc2x0 C. In this case, the rare earth-iron-based rapidly quenched and solidified flakes contain 13 to 15 atomic percent Nd and/or Pr, 5 to 10 atomic percent B, 0 to 20 atomic percent Co, and a balance of an amorphous phase containing impurities and/or an RE2TM14M (RE: Nd or Pr; TM: Fe or Co) phase smaller than 300 nm.
It is preferred that the fully dense magnets be heated for directly energizing the rare earth-iron-based rapidly quenched and solidified flakes. By so doing, the outer peripheral surface of the magnetic pole other than the circumferentially opposite end portions of each of the fully dense magnets and a soft magnetic back yoke are integrated by direct energization.
The circumferentially opposite end portions of each of the fully dense magnets are urged and fixedly retained against the soft magnetic frame by respective springs.
In another aspect of the present invention, a permanent magnet field-type compact DC motor comprises a soft magnetic frame, a pair of permanent magnets fixed to an internal surface of the soft magnetic frame and each having circumferentially opposite end portions, and an armature mounted in the soft magnetic frame and interposed between the pair of permanent magnets, wherein demagnetization curves at the circumferentially opposite end portions are made smaller than a demagnetization curve at a central portion of a magnetic pole of each of the permanent magnets.
The circumferentially opposite end portions are unsaturation-magnetized. The permanent magnets are rare earth magnets mainly composed of rare earth-iron-based rapidly quenched and solidified flakes. The permanent magnets are obtained by compression molding and have a thickness smaller than 1 mm.
The permanent magnet field-type compact DC motor according to the present invention is incorporated into a disc feeder, a pickup device or the like.
As described above, the present invention makes use of the rare earth-iron-based rapidly quenched and solidified flakes, which have a nature in which both the remanent magnetic flux density Br and the coercive force Hc gradually increase simultaneously as functions of a magnetization field, and a well-balanced demagnetization curve can be obtained even in an unsaturation-magnetized state. By combining unsaturation magnetization of field magnets and a conventional cogging torque reducing means offered by a specific magnet shape, a permanent magnet field-type DC motor having a further reduced size, a high output, and a high rotating performance can be obtained.